Experimental and theoretical investigation on the role of friction in Nakazima testing

Abstract

The Nakazima test is a commonly used method to experimentally measure the forming limit curve (FLC) of sheet metal. Various researchers have different opinions about the effect of tribological conditions at the interface of the punch and specimen in Nakazima testing, such that international industry test standards specify conflicting requirements to address friction conditions during testing. This paper provides analysis to resolve this conflict and demonstrates the effectiveness of an analytical procedure to account for and eliminate effects of friction in determination of the strain FLC as a material property that is independent of friction. With the aid of digital image correlation (DIC) techniques, Nakazima tests of a dual phase steel (DP980) were performed under two clearly different friction conditions: (1) friction was minimized between the punch and the specimen by conducting tests using a thin Teflon sheet, and (2) friction was maximized by conducting tests without any lubricant after chemically cleaning the punch and sheet surfaces. As expected, the strain FLCs obtained under these two friction conditions are different. However, analysis shows that the strain path at material points within the same test specimen are the same for both friction conditions. The only difference observed is that the relative strain rates at different material points are strongly affected by friction, which causes necking and fracture to initiate at different locations on test specimens. Furthermore, by accounting for the effects of local strain path, curvature, and contact pressure on the measured strain history at the location of necking using the method described by Min et al. [27], the corrected strain FLCs obtained under very low and very high friction conditions are shown to be indistinguishable.